Direct Observation and Anisotropy of the Contribution of Gap nodes in the Low Temperature Specific Heat of YBa2Cu3O7

The specific heat due to line nodes in the superconducting gap of YBa2Cu3O7 has been blurred up to now by magnetic terms of extrinsic origin, even for high quality crystals. We report the specific heat of a new single crystal grown in a non-corrosive BaZrO3 crucible, for which paramagnetic terms are reduced to ≈ 0.006% spin-1/2 per Cu atom. The contribution of line nodes shows up directly in the difference C(B,T ) − C(0, T ) at fixed temperatures (T < 5 K) as a function of the magnetic field parallel to the c-axis (B ≤ 14 T). These data illustrate the smooth crossover from C ∝ T 2 at low fields to C ∝ TB1/2 at high fields, and provide new values for gap parameters which are quantitatively consistent with tunneling spectroscopy and thermal conductivity in the framework of dx2−y2 pairing symmetry. Data for B along the nodal and antinodal directions in the ab-plane are also provided. The in-plane anisotropy predicted in the clean limit is not observed. Typeset using REVTEX